Surface second harmonic generation (SHG) is a coherent, nonlinear optical technique that is well suited for investigations of biomolecular interactions at interfaces. SHG is surface specific due to the intrinsic symmetry constraints on the nonlinear process, providing a distinct analytical advantage over linear spectroscopic methods, such as fluorescence and UV-Visible absorbance spectroscopies. SHG has the ability to detect low concentrations of analytes, such as proteins, peptides, and small molecules, due to its high sensitivity, and the second harmonic response can be enhanced through the use of target molecules that are resonant with the incident (ω) and/or second harmonic (2ω) frequencies. This review describes the theoretical background of SHG, and then it discusses its sensitivity, limit of detection, and the implementation of the method. It also encompasses the applications of surface SHG directed at the study of protein-surface, small-molecule-surface, and nanoparticle-membrane interactions, as well as molecular chirality, imaging, and immunoassays. The versatility, high sensitivity, and surface specificity of SHG show great potential for developments in biosensors and bioassays.

An extensive investigation into the initial association of HIV-1 Gag with lipid membranes was conducted with second harmonic generation. The roles of the lipid phase, phospholipid 1,2-dioleoyl-sn-glycero-3-phospho-(1-myo-inositol-4,5-bisphosphate) [PI­(4,5)­P2], the presence of the myristoyl group on Gag, the C-terminus of Gag, and the presence of transfer ribonucleic acid (tRNA) in Gag–membrane association were examined using the physiologically most relevant full-length Gag protein studied thus far. The tighter packing of a bilayer composed of gel-phase lipids was found to have a lower relative amount of membrane-bound Gag in comparison to its fluid-phase counterpart. Rather than driving membrane association of Gag, the presence of PI­(4,5)­P2 and the myristoyl group were found to anchor Gag at the membrane by decreasing the rate of desorption. Specifically, the interaction with PI­(4,5)­P2 allows Gag to overcome electrostatic repulsion with negatively charged lipids at the membrane surface. This behavior was verified by measuring the binding properties of Gag mutants in the matrix domain of Gag, which prevented anchoring to the membrane either by blocking interaction with PI­(4,5)­P2 or by preventing exposure of the myristoyl group. The presence of tRNA was found to inhibit Gag association with the membrane by specifically blocking the PI­(4,5)­P2 binding region, thereby preventing exposure of the myristoyl group and precluding subsequent anchoring of Gag to the membrane. While Gag likely samples all membranes, only the anchoring provided by the myristoyl group and PI­(4,5)­P2 allows Gag to accumulate at the membrane. These quantitative results on the kinetics and thermodynamics of Gag association with lipid membranes provide important new information about the mechanism of Gag–membrane association.

The binding interaction between the DNA repair enzyme apurinic/apyrimidinic endonuclease-1 (APE1) with promoter G-quadruplex (G4) folds bearing an abasic site (AP) can serve as a gene regulatory switch during oxidative stress. Prior fluorescence-based analysis in solution suggested APE1 binds the VEGF promoter G4 but whether this interaction was specific or not remained an open question. Second harmonic generation (SHG) was used in this work to measure the noncanonical DNA–protein binding interaction in a label-free assay with high sensitivity to demonstrate the interaction is ordered and specific. The binding of APE1 to the VEGF promoter G4 with AP sites modeled by a tetrahydrofuran analogue produced dissociation constants of ∼100 nM that differed from duplex and single-stranded DNA control studies. The SHG measurements confirmed APE1 binds the VEGF G4 folds in a specific manner resolving a remaining question regarding how this endonuclease with gene regulatory features engages G4 folds. The studies demonstrate the power of SHG to interrogate noncanonical DNA–protein interactions providing a foundational example for the use of this analytical method in future biochemical analyses.

Small-molecule detection in an immunoassay format generally employs competition or labeling. A novel direct-detection label-free primary immunoassay utilizing second harmonic generation (SHG) has been developed and the utility of the method has been demonstrated for several small-molecule narcotics. Specifically, the binding of morphine, methadone, and cocaine to antimorphine, antimethadone, and anticocaine antibodies was measured by SHG, allowing binding affinities and rates of dissociation to be obtained. The SHG primary immunoassay has provided the first kinetic measurements of small-molecule hapten interactions with a receptor antibody. The kinetics reveal for the first time that competitive immunoassays achieve their selectivity by taking advantage of the kinetics of association and dissociation of the labeled and unlabeled target and nontarget small-molecule to the capture antibody. In particular, the induced fit of the target small-molecule to their antibody pairs prolongs their residence time, while the nontarget small-molecule dissociate rapidly in comparison.